The present invention relates to a method of removing a damage layer remaining on an SOI (silicon on insulator) layer after delamination and surface roughness, and simplifying processes in so-called hydrogen ion delamination method (also called smart-cut method) wherein an ion-implanted wafer is bonded to another wafer and then a portion of the ion-implanted wafer is delaminated to provide an SOI wafer.
Recently, public attention has been newly drawn to a method of fabricating an SOI wafer in which an ion-implanted wafer is bonded to another wafer and then a portion of the ion-implanted wafer is delaminated to provide an SOI wafer (hydrogen ion delamination method: so-called smart-cut method). In this method, an oxide film is formed on at least one of two silicon wafers; hydrogen ions or rare gas ions are implanted into the upper surface of one of the wafers in order to form a fine bubble layer (enclosed layer) within the wafer; the ion-implanted silicon wafer is superposed on the other silicon wafer such that the ion-implanted surface comes into close contact with the other silicon wafer via the oxide film; heat treatment (delaminating heat treatment) is then performed to delaminate a portion of one of the wafers using the fine bubble layer as a delaminating plane, in order to form a thin film; and heat treatment (bonding heat treatment) is further performed to firmly bond them, to provide an SOI wafer (see Japanese Patent Application Laid-Open (Kokai) No. 5-211128). According to this method, since a split surface (a delaminated surface) is a good mirror-like surface, and an SOI wafer whose SOI layer has a high thickness uniformity can be obtained relatively easily.
However, when the SOI wafer is fabricated according to the hydrogen ion delamination method, there exists a damage layer due to ion implantation on the surface of the SOI wafer after delamination, and surface roughness of the resultant SOI wafer is large compared with a mirror surface of a general silicon wafer. Accordingly, it is necessary to remove such a damage layer and surface roughness in the hydrogen ion delamination method. Conventionally, in order to remove a damage layer or the like there has been performed as the final step after bonding heat treatment a mirror polishing process, called xe2x80x9ctouch polishingxe2x80x9d, wherein a stock removal is very small.
However, when the SOI layer is polished by means of mechanical machining, thickness uniformity of the SOI layer achieved by hydrogen ion implantation and delamination is degraded, since the stock removal is not uniform. Moreover, if mirror polishing is conducted after bonding heat treatment, it involves many and complicated processes and causes disadvantage in cost.
There has been thus proposed a method of removing a damage layer, by so-called sacrificial oxidation wherein an oxide film on an SOI layer by subjecting the SOI layer after delamination to heat treatment in an oxidizing atmosphere and then removing the oxide film. According to the method, a damage layer can be removed without performing polishing by means of mechanical machining.
However, surface roughness of the SOI layer cannot be sufficiently improved only with the above-mentioned sacrificial oxidation. Accordingly, it is still necessary to perform touch polishing which is mechanical polishing in order to improve surface roughness, which may lead to degradation of thickness uniformity of the SOI layer. Furthermore, when heat treatment is performed in an oxidizing atmosphere, OSFs (oxidation induced stacking faults) are sometimes generated due to damage on the surface of the SOI layer.
There has been thus proposed a method for removing a damage on the SOI layer and improving surface roughness by performing heat treatment in a reducing atmosphere without polishing the surface of the SOI wafer obtained by the hydrogen ion delamination method. According to the method, the damage remaining on the SOI layer after delamination and surface roughness of the surface of the SOI layer can be improved, with maintaining thickness uniformity.
However, damage in the SOI layer of the SOI wafer obtained by hydrogen ion delamination method is large in a surface part, and gets smaller at the inner part of the layer. When heat treatment in a reducing atmosphere is performed as described above, removal of damage will progress from the inner part to the surface part of the SOI layer. However, if the damage in a surface part is large, heat treatment at high temperature for a long time will be necessary, and in some cases, damage is not removed completely even when heat treatment is performed at high temperature for a long time.
Size and depth of the damage depend on the degree of implantation energy and a dose amount of hydrogen ion implantation. Accordingly, such a problem is serious especially, for example, in the case that large implantation energy is required for producing an SOI wafer having a thick SOI layer or a thick buried oxide layer, or in that case that it is necessary to increase a doze amount for the purpose of performing delaminating heat treatment at a low temperature.
When heat treatment in a reducing atmosphere such as hydrogen atmosphere or the like is performed at high temperature for a long time, silicon on the surface of the SOI layer is etched, so that thickness uniformity is degraded, and etch pits are sometimes generated in the buried oxide layer. These problems are caused, when the SOI layer includes defects such as COPs (Crystal Originated Particle) that extend to the oxide film thereunder. Because, the COPs are not eliminated but remain, or sometimes enlarged, and therefore the buried oxide layer may also be etched by hydrogen or the like getting therein through the defects, and pits are formed there. The etch pits cause a problem that the SOI layer near them are affected by them.
As described above, there have been proposed various methods in order to remove a damage layer and surface roughness of an SOI wafer obtained by hydrogen ion delamination method maintaining thickness uniformity of an SOI layer. However, they are not satisfactory, and appropriate methods for solving the problems have been required.
The present invention has been accomplished to solve the above-mentioned problems. An object of the present invention is to provide a method of fabricating a high quality SOI wafer by hydrogen ion delamination method wherein a damage layer remaining on the surface of the SOI layer after delamination and surface roughness are removed maintaining thickness uniformity of the SOI layer, and to improve productivity in manufacture of wafer.
To achieve the above mentioned object, the present invention provides a method of fabricating an SOI wafer by hydrogen ion delamination method wherein an oxide film is formed on an SOI layer by heat treatment in an oxidizing atmosphere after bonding heat treatment, then the oxide film is removed, and subsequently heat treatment is performed in a reducing atmosphere.
When the SOI layer is subjected to sacrificial oxidation wherein the oxide film is formed on the SOI layer by heat treatment in an oxidizing atmosphere after bonding heat treatment and then the oxide film is removed as described above, all or a part of a damage layer on the surface of the SOI layer is incorporated into the oxide film. Accordingly, when the oxide film is removed, the damage layer can be efficiently removed. And, when it is subsequently subjected to heat treatment in a reducing atmosphere, the damage layer remaining on the SOI layer can be recovered, and surface roughness can be improved. In that case, since all or a part of the damage layer on the SOI layer has already been removed, heat treatment time can be short, so that the SOI layer or a buried oxide layer can be prevented from being etched. Moreover, the method does not require mechanical polishing, and thus thickness uniformity of the SOI layer is not degraded. As a result, an SOI wafer having extremely high quality can be fabricated by the hydrogen ion delamination method in higher productivity.
The present invention also provides a method of fabricating an SOI wafer by hydrogen ion delamination method wherein an oxide film is formed on an SOI layer by heat treatment in an oxidizing atmosphere after delaminating heat treatment, then the oxide film is removed, and subsequently heat treatment is performed in a reducing atmosphere.
According to this method, the oxide film is formed on the SOI layer by heat treatment in an oxidizing atmosphere after delaminating heat treatment and then the oxide film is removed as described above, and therefore, all or a part of a damage layer on the surface of the SOI layer is incorporated into the oxide film. Accordingly, when the oxide film is removed, the damage layer can be efficiently removed. And, when it is subjected to heat treatment in a reducing atmosphere, the damage layer remaining on the SOI layer can be recovered, and surface roughness can be improved. In that case, since all or a part of the damage layer on the SOI layer has already been removed, heat treatment can be performed in short time and effectively, so that the SOI layer or a buried oxide layer can be prevented from being etched. Moreover, the method does not require polishing that is mechanical machining, and thus thickness uniformity of the SOI layer is not degraded.
In the above method, if bonding heat treatment is performed after the oxide film is formed, the oxide film can function also as a surface protecting film during the bonding heat treatment. Thus, the surface of the SOI layer can be prevented from being etched, differently from the case that the bonding heat treatment is performed in non-oxidizing atmosphere. Alternatively, heat treatment in an oxidizing atmosphere or a heat treatment in the reducing atmosphere can also function as bonding heat treatment. In that case, it is not necessary to perform bonding heat treatment independently, so that the SOI wafer having extremely high quality can be fabricated by hydrogen ion delamination method in more simple processes and in high productivity.
In that case, the above-mentioned heat treatment in a reducing atmosphere is preferably performed at a temperature in the range from 1000xc2x0 C. to 1300xc2x0 C. for 1 to 60 seconds through use of a rapid heating/rapid cooling apparatus.
When heat treatment in a reducing atmosphere is performed at a high temperature in the range from 1000xc2x0 C. to 1300xc2x0 C. for a short time of 1 to 60 seconds through use of a rapid heating/rapid cooling apparatus as described above, a damage layer on the surface of the SOI wafer and surface roughness can be efficiently improved in an extremely short time. Furthermore, the SOI layer or the buried oxide layer can be prevented from being etched.
The above-mentioned heat treatment in a reducing atmosphere is preferably performed in 100% hydrogen atmosphere or a mixed atmosphere of hydrogen and argon.
If heat treatment is performed in such an atmosphere, a damage layer on the surface of the SOI layer and the surface roughness can be surely improved.
Thickness of the oxide film on the SOI layer formed by the above-mentioned heat treatment in an oxidizing atmosphere can be twice or more the thickness of the damage layer existing on the surface of the SOI layer before the oxide film is formed.
As described above, when the thickness of the oxide film formed on the SOI layer by heat treatment in an oxidizing atmosphere is twice or more the thickness of the damage layer existing on the surface of the SOI layer before the oxide film is formed, almost all of the damage layer can be incorporated into the formed oxide film, so that heat treatment time to be performed later in a reducing atmosphere can be shorter. Accordingly, the damage layer or the like can be removed efficiently.
Moreover, temperature for the above-mentioned heat treatment in an oxidizing atmosphere is preferably lower than temperature for the above-mentioned heat treatment in an reducing atmosphere for the following reason.
OSFs are sometimes generated due to the damage on the surface of the SOI layer while heat treatment in an oxidizing atmosphere is performed. If heat treatment in a reducing atmosphere is performed at a higher temperature than temperature for heat treatment in an oxidizing atmosphere, the OSFs generated on the surface of the SOI layer can be easily removed.
The above-mentioned heat treatment in an oxidizing atmosphere is preferably performed at a temperature of 1000xc2x0 C. or lower.
As described above, if heat treatment in an oxidizing atmosphere is performed at a temperature of 1000xc2x0 C. or lower, generation of OSFs on the SOI layer can be prevented.
The SOI wafer fabricated according to the method of the present invention has no damage layer on the SOI layer, and has improved surface roughness, good thickness uniformity and excellent quality.
As described above, according to the present invention, a damage layer remaining on the surface of the SOI layer and surface roughness can be removed efficiently, maintaining thickness uniformity of the SOI layer by performing a combination of sacrificial oxidation and heat treatment in a reducing atmosphere after delamination in a hydrogen ion delamination method. Accordingly, SOI wafer having extremely high quality can be produced in high productivity.